30 PRESIDENT’S ADDRESS—SECTION A. 
the plates, and the condenser will return more or less nearly to 
its uncharged condition. Faraday found that when different 
dielectrics are placed between the metallic plates of the con- 
denser, the sparks occurring on discharge differ in intensity 
according to the nature of the dielectric. A simple experiment 
will also show that the energy appearing at discharge is stored in 
the dielectric itself. Consequently we must admit that different 
dielectrics will, under corresponding electrical circumstances, 
store different amounts of energy. A vacuum seems to be a 
dielectric and so the power of storing energy is equally possessed 
by it, and that, by the way, to a degree hardly inferior to that 
possessed by air and other gases. If the dielectric is a fluid like 
benzene or turpentine, and one of the condenser-plates is fixed 
while the other is hung from the arm of a balance, the plates 
being charged to a given potential, we shall find that very 
different sets of weights are necessary to overcome the corres- 
ponding attraction in different dielectrics. Since the power of 
preventing the passage of sparks is very different in different 
media, we will suppose that our plates have an attachment 
whereby the sparks are made always to occur in air. The 
distances at which sparks will occur between surfaces of a given 
form in air are practically proportional to a quantity perfectly 
detined in the mathematical theory and called the electromotive 
force. 
Now, let us charge our condenser-plates and then connect 
them by a wire; we shall find that the plates are discharged and 
that the phenomena of a current, as it is called, are exhibited in 
the wire during the discharge. If the wire encircles a magnetic 
needle, for instance, the needle will be detlected ; or, if a volta- 
meter be included, decomposition, say of water, will take place 
in it. Before the discharge the dielectric is said to suffer 
“polarisation,” or, as Maxwell called it, “ electric displacement.” 
The former term is the better, because all that was meant by 
Maxwell, at all events in his later work, was that the dielectric 
suffered a vector or directed change of some sort; while the term 
he used is apt to connote the actual transference of something— 
an idea totally at variance with the theory. Since, as I said, a 
vacuum acts as well, or nearly so, as anything else, we must 
assume that the ether is the vehicle in which the polarisation 
occurs, and that in dielectrics other than vacua the properties of 
this ether are so modified as to account for the comparatively 
slight differences of effect observed. The second point of 
Maxwell’s theory is that the dielectric polarisation is precisely 
proportional to the electromotive force. This is not all, however, 
Maxwell made a further assumption, immensely facilitating 
mathematical computation, and justified as we shall see by 
the consistency of the results to which it leads, and by their 
concordance with experiment. He supposed that the polarisation 
